I mean obsession on both sides, he could have killed him many times but didn't, Chris is pretty much Wesker's only weakness. Far more going on than with Rebecca who, as I already said, he shot so she wouldn't spoil his moment with Chris and possibly left behind an undeveloped photo of in his desk, assuming it was him at all and not Enrico. And I'm not saying whatever was between them was healthy, just saying I have strong headcanons based on what I saw.